This invention generally relates to a process and apparatus for accomplishing the preparation of thin solid films; and more specifically the present invention is directed to an improved process and apparatus for formulating hydrogenated amorphous silicon photoresponsive imaging member by plasma deposition or glow discharge techniques. In one embodiment the present invention envisions an improved apparatus and process for the preparation of a hydrogenated amorphous silicon photoresponsive imaging member by the rapid plasma deposition of amorphous silicon on a cylindrical substrate with magnets therein by subjecting an appropriates gas source material such as a silane gas to decomposition by an electrical discharge subsequent to permitting the gas to flow toward the cylindrical substrate, preferably in a crossward direction or in a direction orthogonal to the cylinder substrate axis. Incorporation of magnets into the cylinder or electrode component contained in the vacuum chamber enables a more efficient, economical, and uniform deposition of hydrogenated amorphous silicon. Additionally, with the process and apparatus of the present invention there is achieved deposition of hydrogenated amorphous silicon in a rapid sequence, that is, for example in some instances there is a doubling of the deposition rate as compared to some prior art processes. Thereafter, the cylindrical substrate containing on its surface deposited hydrogenated amorphous silicon is removed from the vacuum chamber, and can be selected as a photoconductor for incorporation into electrostatographic imaging apparatuses inclusive of electrostatic imaging and printing devices.
Electrostatographic imaging processes with photoresponsive or photoconductive materials are well known. Examples of photoconductors selected include amorphous selenium, alloys of selenium such as selenium tellurium, selenium arsenic, and the like. Additionally, layered photoresponsive imaging members can be selected for electrostatic imaging systems including those containing therein photogenerating substances and charge transport molecules, reference for example U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference. Additionally, the use of hydrogenated amorphous silicon as a photoreceptor material is illustrated in the prior art. Apparently, hydrogenated amorphous silicon possesses a number of advantages in comparison to, for example, amorphous selenium-based materials in that amorphous silicon is of extreme hardness and will not crystallize over extended time periods, even at temperatures as high as several hundred degrees Centigrade. Additionally, amorphous silicon photoconductors have excellent photoelectric properties, high absorption coefficients through the visible region, and are relatively low in useful life cost in comparison to selenium photoconductors, for example. Moreover, amorphous silicon members are capable of ambipolarity as they can be xerographically charged and discharged either positively or negatively in various imaging systems. Further, amorphous silicon can be modified by adding various dopants thereto, such as boron and phosphorus enabling this material to function as p on n type semiconductor devices. Also, amorphous silicon may be alloyed with other elements, such as germanium and tin for the purpose of providing a material which will be photosensitive in the infrared region.
Hydrogenated amorphous silicon, which is classified as a tetrahedrally bonded amorphous semiconductor, can be prepared by known thermal evaporation techniques similar to those selected for the preparation of selenium photoconductor. However, amorphous silicon prepared in accordance with such a process usually has a relatively high dark conductivity, about 10.sup.-3 ohm-cm(.sup.-1), thereby causing the disappearance of any charge from the surface thereof resulting in a substantially useless member for xerographic imaging purposes. Amorphous silicon prepared by the grow discharge of the gas silane results in a material having a much lower conductivity, such as below 10.sup.-9 ohm-cm(.sup.-1) reference for example the Journal of Electrochemical Society, vol. 116, page 77, (1969), Chittick et al; and the Journal of Noncrystallline Solids, Volume 3, pages 255-270, (1970), Chittick et al.. Apparently, in these processes, hydrogen atoms saturate dangling silicon bonds and remove the band gap states causing the Fermi level to move towards mid-gap.
It is believed that the dangling bonds intrinsically incorporated in amorphous silicon can be reduced, and to some extend, controlled by the choice of film deposition conditions. These dangling bonds are apparently present in a density of sufficient magnitude to render films of amorphous silicon prepared by thermal evaporation techniques or sputtering processes unsuitable for semiconductor and photoelectronic purposes. For example, the resulting material cannot be successfully doped and used to produce p or n type operative devices. Further, intrinsic dangling bonds function as recombination sites rendering the resulting film substantially useless for photoelectronic devices. In those situations wherein, for example, amorphous silicon films are prepared in the presence of a reactive gas, the undesirable localized states are removed from the band gas as a result of the intrinsic dangling bond defects being coordinated with fragments of the reactive gas. Examples of reactive gases selected for coordination with the intrinsic silicon dangling bonds include hydrogen gas and fluorine gas.
Accordingly, thermally evaporated silicon generally must contain, for example, hydrogen or fluorine in order to be useful as a photoconductive member. Thus, the preparation of such a material requires processes and apparatus which are vastly different from those required for the familiar thermal evaporation techniques selected for the preparation of chalcogenides.
One known method for obtaining amorphous silicon materials is referred to in the art as the glow discharge process. In this process, the vapor deposition of a silane gas occurs by causing the gas to flow between two electrodes, one of which has a substrate contained therein. As electrical power is applied to the electrodes, the silane gas decomposes into a reactive silicon hydrogen species, which will deposit as a solid film on both electrodes. The presence of hydrogen can be of critical importance since it tends to coordinate with the dangling bonds in the silicon in part, as the mono, di, and trihydrides, thereby serving to passivate these dangling bonds.
Accordingly, thermally evaporated silicon generally must contain, for example, hydrogen fo fluorine in order to be useful as a photoconductive member. Thus, the preparation of such a material requires processes and apparatus which are vastly different from those required for the familiar thermal evaporation techniques selected for the preparation of chalcogenides.
One known method for obtaining amorphous silicon materials is referred to in the art as the glow discharge process. In this process, the vapor deposition of a silane gas occurs by causing the gas to flow between two electrodes, one of which has a substrate contained thereon. As electrical power is applied to the electrodes, the silane gas decomposes into a reactive silicon hydrogen species, which will deposit as a solid film on both electrodes. The presence of hydrogen can be of critical importance since it tends to coordinate with the dangling bond in the silicon in part, as the mono, di, and trihydrides, thereby serving to passivate these dangling bonds.
In another known process, amorphous silicon can be prepared by a sputtering technique wherein a substrate is attached to one electrode, and a target of silicon is placed on a second electrode. These electrodes are connected to a high voltage power supply and a gas which is usually a mixture of argon, and hydrogen is introduced between the electrodes to provide a medium in which a glow discharge or plasma can be initiated and maintained. The glow discharge provides ions which strike the silicon target, and cause the removal by momentum transfer of mainly neutral target atoms, which subsequently condense as a thin film on the substrate electrode. Also, the glow discharge functions to activate the hydrogen causing it to react with the silicon, and be incorporated into the deposited silicon film. The activated hydrogen also coordinates with the dangling bonds of the silicon to form mono, di, and trihydrides.
There is also known an apparatus and process for preparing amorphous silicon films on a substrate, which involves means for directing and accelerating an ion beam from a plasma toward a sputtering target contained within a chamber, which chamber also contains a shield means having a low sputtering efficiency compared to the sputtering target. The shield means is situated between stray ion beams and the vacuum chamber surface. More specifically, the ion beam process involves producing semiconductive films on a substrate comprising generating the plasma; directing and accelerating an ion beam of the plasma toward a sputtering target, the target being contained in a vacuum chamber at reduced pressure; shielding the vacuum chamber surface from stay ion beams, whereby sputtering of the vacuum chamber surface by the plasma is minimized, followed by sputtering the target with the ion beam to sputter the target material; and collecting the sputtered target material as a film on the substrate, the substrate being physically isolated from the plasma generating process and the sputtering process.
While the latter processes may be suitable for their intended purposes, they suffer from a number of disadvantages, including for example, very low rates of material deposition, the inability to obtain uniform coatings over large areas, and the inability to form multilayers without moving or removing the target and/or the substrate.
Additionally, there is disclosed in U.S. Pat. No. 4,265,991 an amorphous silicon photoconductor. This patent describes several processes for preparing amorphous silicon. In one process, there is prepared an electrophotographic photosensitive member which involved heating the electrophotographic member contained in a chamber to temperature of 50.degree. C. to 350.degree. C., introducing a gas containing a hydrogen atom into the deposition chamber, causing an electrical discharge in the space of the deposition chamber in which a silicon compound is present, by electric energy to ionize the gas, followed by depositing amorphous silicon on the electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second by utilizing an electric discharge while raising the temperature of the substrate thereby resulting in a amorphous silicon photoconductive layer of a predetermined thickness.
Further, there is illustrated in U.S. Pat. No. 4,513,022, the disclosure of which is totally incorporated herein by reference, a process and apparatus for formulating hydrogenated amorphous silicon photoconductive members. Thus, there is described in this patent an apparatus for preparing hydrogenated amorphous silicon imaging members comprised of a first electrode means; a second counterelectrode means; a receptable means for the first electrode means and the second counterelectrode means; a substrate means to be coated contained on the first electrode means; which substrate is in the form of a cylindrical member, a gas inlet means, and a gas exhaust means, wherein a silane gas is introduced into the receptable in a crossflow direction perpendicular to the axis of the cylindrical member. The process and apparatus of the present application is similar to that disclosed in the aforementioned patent with the primary exception that with the apparatus of the present application there is included in the cylindrical member permanent magnets thereby permitting the advantages as detailed hereinbefore inclusive of increased deposition rates for the hydrogenated amorphous silicon.
Therefore, there is a need for improved processes and apparatuses for obtaining new films of photoconductive amorphous silicon at increased deposition rates over large areas. Additionally, there continues to be a need for improved processes and apparatuses for formulating hydrogenated amorphous silicon photoconductive members which processes and apparatuses are simple in design, economically attractive and which are susceptible to a batch process. Also, there is a need for processes and apparatuses for obtaining hydrogenated amorphous silicon at deposition rates of greater than 1 micron per hour, and wherein there results uniformity in thickness. Further, there is a need for a more efficient economical process for the preparation of hydrogenated amorphous silicon photoconductive imaging members. Moreover, there continues to be a need for processes and apparatuses for preparing amorphous silicon photoconductive structures having superior mechanical strength, improved chemical stability, and substantially no toxicity problems associated therewith. Further, there continues to be a need for improved processes and apparatuses for obtaining amorphous silicon photoconductors wherein the thickness of the resulting film on a substrate will be axially and radially uniform over a variety of operating conditions. Also, there continues to be a need for improved processes and apparatuses for obtaining amorphous silicon photoconductors wherein virtually all the silane source gas material is converted to amorphous silicon. Additionally, there continues to be a need for improved apparatus for the deposition of amorphous silicon photoconductors, wherein the apparatus can be easily and expeditiously cleaned between depositions to avoid a buildup of material in the deposition chamber. Finally, there continues to be a need for a process and an apparatus which are suitable for the deposition of amorphous silicon and related materials on numerous cylindrical substrates contained in one deposition apparatus such as a vacuum chamber.